1. Field of the Invention
The present invention relates to an echo canceling apparatus in which an echo occurring on a near-end caller""s side is canceled in a system in which a speech communication is performed between the near-end caller and a far-end caller through a transmission path.
2. Description of Related Art
FIG. 10 is a block diagram showing the configuration of a conventional echo canceling apparatus disclosed in the Published Unexamined Japanese Patent Application No. H10-242891 (1998) as an example. In FIG. 10, 1 indicates a reception signal received from a far-end caller, 2 indicates a transmission signal transmitted from a near-end caller to the far-end caller, 3 indicates a first adaptive filter unit, 4 indicates a first subtracting unit, 5 indicates a second adaptive filter unit, 6 indicates a second subtracting unit, 7 indicates an adaptive filter control unit, 8 indicates a filter selecting unit, 9 indicates a, tap coefficient storing unit, 10 indicates a pseudo-echo producing unit, 11 indicates a third subtracting unit, 12 indicates a delaying unit, 13 indicates a delaying unit, and 14 indicates an echo canceled output signal.
Next, an operation of a conventional echo canceling apparatus is described.
In this apparatus, a group of first tap coefficients, a group of second tap coefficients and a group of third tap coefficients are used. The group of first tap coefficients is renewed by using a first convergence factor in the first adaptive filter unit 3 for each sample cycle. Also, a first pseudo-echo signal is produced by using the group of first tap coefficients and the reception signal 1 in the first adaptive filter unit 3 and is output to the first subtracting unit 4. In the first subtracting unit 4, the first pseudo-echo signal is subtracted from the transmission signal 2 to produce a first error signal, and the first error signal is output to the adaptive filter control unit 7. Also, the first error signal is returned to the first adaptive filter unit 3 to renew the group of first tap coefficients. The group of first tap coefficients of the first adaptive filter unit 3 is renewed according to an equation (1).
(Hk,j+1)=(Hk,j)+xcexcxc3x97E1jxc3x97{Xjxe2x88x92k}/xcex1xe2x80x83xe2x80x83(1)
Here the symbol k denotes a tap number (k=0,1, - - - ,N), the symbol j denotes a j-th time point, the symbol Hk,j denotes a tap coefficient of a k-th tap at the j-th time point, and the symbol Hk,j+1 denotes a tap coefficient of a k-th tap at the (j+1)-th time point. In other words, the symbol Hk,j+1 denotes a tap coefficient at a sample time point next to that of the tap coefficient Hk,j. Also, the symbol E1j denotes a value of the first error signal at the j-th time point, and the symbol Xjxe2x88x92k denotes the reception signal 1 at the (jxe2x88x92k)-th time point. The symbol xcexc is called a convergence factor and denotes a parameter for determining a convergence speed of each tap coefficient. In cases where the convergence factor xcexc is heightened, the convergence speed of the group of first tap coefficients is heightened. In contrast, in cases where the convergence factor xcexc is lowered, the convergence speed of the group of first tap coefficients is lowered. A factor xcex1 is indicated according to an equation (2).                     α        =                              ∑                          i              =              0                        N                    ⁢                                    "LeftBracketingBar"              Xi              "RightBracketingBar"                        /            N                                              (        2        )            
The symbol N denotes the number of tap coefficients.
The second group of tap coefficients is renewed by using a second convergence factor lower than the first convergence factor in the second adaptive filter unit 5 for each sample cycle according to the equation (1) in the same manner as the renewal of the group of first tap coefficients. Also, a second pseudo-echo signal is produced by using the group of second tap coefficients and the reception signal 1 in the second adaptive filter unit 5 and is output to the second subtracting unit 6. In the second subtracting unit 6, the second pseudo-echo signal is subtracted from the transmission signal 2 to produce a second error signal, and the second error signal is output to the adaptive filter control unit 7. Also, the second error signal is returned to the second adaptive filter unit 5 to renew the group of second tap coefficients.
Also, one group of tap coefficients is selected in the filter selecting unit 8 from the group of first tap coefficients of the first adaptive filter unit 3, the group of second tap coefficients of the second adaptive filter unit 5 and a group of tap coefficients stored in the tap coefficient storing unit 9, and the selected group of tap coefficients is stored in the tap coefficient storing unit 9 as a group of third tap coefficients.
In the adaptive filter control unit 7, the first error signal, the second error signal, the reception signal 1 and the transmission signal 2 are input for each sample cycle (or each sample period), an average power level of M samples of each signal is calculated for each tap coefficient selection cycle corresponding to M sample cycles, selection information is determined according to equations (3) to (6) and four selection conditions, and the selection information is output to the filter selecting unit 8 to select one group of tap coefficients from the group of first tap coefficients, the group of second tap coefficients and the group of third tap coefficients of the tap coefficient storing unit 9.
X less than p1xe2x80x83xe2x80x83(3)
S greater than p2xc3x97X (here, p2xe2x89xa60.5)xe2x80x83xe2x80x83(4)
P3xc3x97|E1| greater than |E2|xe2x80x83xe2x80x83(5)
P4xc3x97|E2| greater than |S|xe2x80x83xe2x80x83(6)
Here the symbol S denotes an average power level (which indicates a root-mean-square level or an average amplitude level) of the transmission signal 2, the symbol X denotes an average power level of the reception signal 1, the symbol E1 denotes an average power level of the first error signal, and the symbol E2 denotes an average power level of the second error signal. Also, the symbols p1, p2, p3 and p4 are respectively a constant and are determined according to environmental conditions of the conventional echo canceling apparatus.
The selection conditions of one group of tap coefficients using the equations (3) to (6) are as follows.
First Selection Condition: the equation (3) is satisfied, or the equation (4) is satisfied on condition that the equation (3) is not satisfied. In this case, the group of third tap coefficients of the tap coefficient storing unit 9 is selected.
Second Selection Condition: the equation (5) is satisfied on condition that either the equation (3) or the equation (4) is not satisfied. In this case, the group of first tap coefficients is selected.
Third Selection Condition: the equation (6) is satisfied on condition that each of the equations (3), (4) and (5) is not satisfied. In this case, the group of second tap. coefficients is selected.
Fourth Selection Condition: each of the equations (3), (4), (5) and (6) is not satisfied. In this case, the group of third tap coefficients of the tap coefficient storing unit 9 is selected.
In the pseudo-echo producing unit 10, a third pseudo-echo signal is produced by using the group of third tap coefficients stored in the tap coefficient storing unit 9 and the reception signal 1 delayed in the delaying unit 13, and the third pseudo-echo, signal is output to the third subtracting unit 11. In the third subtracting unit 11, the third pseudo-echo signal is subtracted from the transmission signal 2 delayed in the delaying unit 12 to produce an echo canceled output signal 14, and the echo canceled output signal 14 is output to the far-end caller.
Echo canceling apparatuses similar to the above-described conventional echo canceling apparatus are disclosed in the Published Unexamined Japanese Patent Application No. H9-148965 (1997) and the Published Unexamined Japanese Patent Application No. H9-181653 (1997).
However, because the conventional echo canceling apparatus has the above-described configuration, there are following problems.
(First Problem)
It is required to determine the value of the constant p2 used in the equation (4) in advance according to environmental conditions of an echo path used in the conventional echo canceling apparatus. Therefore, the control of the adaptive filter control unit 7 is available only for the echo path environmental conditions corresponding to the value of the constant p2.
(Second Problem)
Because the group of tap coefficients is always renewed for each sample cycle in the first adaptive filter unit 3 and the second adaptive filter unit 5, when either the reception signal 1 or the transmission signal 2 is not input, when only the transmission signal 2 is input or when the reception signal 1 and the transmission signal 2 are input (a double-talk), the group of first tap coefficients of the first adaptive filter unit 3 and the group of second tap coefficients of the second adaptive filter unit 5 deteriorate. In particular, in the double-talk, in cases where the deterioration degree of the group of tap coefficients reaches a certain value, there is a possibility that the average power level of the first error signal becomes lower than that of the second error signal. In this case, the second selection condition is satisfied, the deteriorating group of first tap coefficients is undesirably selected, the deteriorating group of first tap coefficients is stored in the tap coefficient storing unit 9 as a deteriorating group of third tap coefficients, and the third pseudo-echo signal output from the pseudo-echo generating unit 10 according to the deteriorating group of third tap coefficients of the tap coefficient storing unit 9 greatly differs from an ideal pseudo-echo signal which cancels an echo included in the transmission signal 1. Therefore, there is a possibility that an echo included in the transmission signal 1 is amplified in the echo canceled output signal 14 on the contrary.
(Third Problem)
In cases where the value M corresponding to the tap coefficient selection cycle is set to a low value, the number of renewal operations for the group of tap coefficients performed in the first and second adaptive filter units 3 and 5 in one tap coefficient selection cycle becomes small. Therefore, the precision for estimating a group of ideal tap coefficients by renewing the group of first tap coefficients becomes almost the same as that for estimating a group of ideal tap coefficients by renewing the group of second tap coefficients, so that the average power levels of the transmission signal 2, the first error signal and the second error signal become almost the same as each other. In this case, either the second selection condition or the third selection condition is not satisfied, but the fourth selection condition is always satisfied. Therefore, the group of third tap coefficients stored in the tap coefficient storing unit 9 is always selected to produce the echo canceled output signal 14, and there is a probability that the estimate of the ideal group of tap coefficients to be used to efficiently cancel an echo existing in the transmission signal 2 is not advanced. In contrast, in cases where the value M is set to a high value to reliably estimate the group of ideal tap coefficients, it is required to prolong a delay time set in the delaying units 12 and 13 according to the value M set to a high value, and the delay time of the echo canceled output signal 14 becomes long. In this case, there is a probability that the far-end caller receiving the echo canceled output signal 14 feels strange in a speech communication with the near-end caller. Also, as the delay time of the echo canceled output signal 14 is lengthened, it is required to enlarge a memory capacity required to store the reception signal 1 and the transmission signal 2 delayed in the delaying units 12 and 13, so that memory sizes of the delaying units 12 and 13 become large.
(Fourth Problem)
It is required to store the of group of first tap coefficients having a large volume, the group of second tap coefficients having a large volume and the group of third tap coefficients having a large volume in the first adaptive filter unit 3, the second adaptive filter unit 5 and the tap coefficient storing unit 9. Therefore, a memory capacity required to store the group of first tap coefficients, the group of second tap coefficients and the group of third tap coefficients becomes large, so that a size of the conventional echo canceling apparatus becomes large.
An object of the present invention is to provide, with due consideration to the problems of the conventional echo canceling apparatus, an echo canceling apparatus in which the degradation of an echo canceling performance is suppressed while giving no delaying time to an echo canceled output signal and making a size of the echo canceling apparatus small.
The object is achieved by the provision of an echo canceling apparatus, in which an echo canceled output signal is produced from a reception signal, a transmission signal, a first error signal produced from the reception signal and the transmission signal and a second error signal produced from the reception signal and the transmission signal and is output, comprising:
first adaptive filter renewal control means for receiving the first error signal, the reception signal and the transmission signal and judging according to a first renewal judging method based on the first error signal, the reception signal and the transmission signal whether or not a first tap coefficient is to be renewed;
first adaptive filter means for receiving the first error signal and the reception signal, renewing the first tap coefficient according to the first error signal and the reception signal, in cases where the renewal of the first tap coefficient is judged by the first adaptive filter renewal control means, and producing a first pseudo-echo signal from the first tap coefficient;
first subtracting means for subtracting the first pseudo-echo signal produced by the first adaptive filter means from the transmission signal to produce the first error signal as the echo canceled output signal equivalent to the transmission signal in which an echo component is canceled;
second adaptive filter renewal control means for receiving the reception signal and judging according to a second renewal judging method, which is based on the reception signal and differs from the first renewal judging method, whether or not a second tap coefficient is to be renewed;
second adaptive filter means for receiving the second error signal and the reception signal, renewing the second tap coefficient according to the second error signal and the reception signal, in cases where the renewal of the second tap coefficient is judged by the second adaptive filter renewal control means, and producing a second pseudo-echo signal from the second tap coefficient;
second subtracting means for subtracting the second pseudo-echo signal produced by the second adaptive filter means from the transmission signal to produce the second error signal equivalent to the transmission signal in which the echo component is canceled; and
adaptive filter selecting means for receiving the reception signal, the transmission signal, the first error signal produced by the first subtracting means, the second error signal produced by the second subtracting means and the second tap coefficient of the second adaptive filter means and controlling the first adaptive filter means according to a level relationship among the reception signal, the transmission signal, the first error signal and the second error signal to replace the first tap coefficient of the first adaptive filter means with the second tap coefficient.
In the above configuration, because the first adaptive filter renewal control means judges according to the first error signal, the reception signal and the transmission signal whether or not the first tap coefficient of first adaptive filter means is to be renewed, the first tap coefficient is stably converged, and a deterioration degree of the first tap coefficient is reduced in a double talk state. In contrast, because the second adaptive filter renewal control means judges according to only the reception signal whether or not the second tap coefficient of second adaptive filter means is to be renewed, though a deterioration degree of the second tap coefficient is high in the double talk state, the second tap coefficient is rapidly converged in a state in which only the echo component exists in the transmission signal.
Therefore, the echo canceled output signal is produced from the first tap coefficient stably converged in the only echo component state. In contrast, in case of a before-initial estimate, an echo, path changes or an infinite return loss, because a rapid convergence of the tap coefficient is required, the adaptive filter selecting means controls the first adaptive filter means to replace the first tap coefficient of the first adaptive filter means with the second tap coefficient, and the echo canceled output signal is produced from the second tap coefficient rapidly converged.
Accordingly, because the echo canceled output signal produced from the first tap coefficient stably converged in the only echo component state can be produced and because the echo canceled output signal produced from the second tap coefficient rapidly converged can be produced in case of the before-initial estimate, an echo path change or an infinite return loss, the echo canceled output signal equivalent to the transmission signal, in which the echo component is canceled according to the highly estimated tap coefficient relating to the production of the echo canceled output signal, is always produced and output regardless of the echo path change or the infinite return loss. Also, the echo canceling apparatus not depending on an echo path environment can be manufactured in a small size without amplifying the echo included in the transmission signal. Also, the echo canceling apparatus, in which the echo canceled output signal is output without any delaying time, can be obtained.
Also, the object is achieved by the provision of an echo canceling apparatus, in which an echo canceled output signal is produced from a reception signal, a transmission signal, a first error signal produced from the reception signal and the transmission signal and a second error signal produced from the reception signal and the transmission signal and is output, comprising:
first adaptive filter renewal control means for receiving the reception signal and judging according to the reception signal whether or not a first tap coefficient is to be renewed;
first adaptive filter means for receiving the first error signal and the reception signal, renewing the first tap coefficient according to a first tap coefficient renewal algorithm based on the first error signal and the reception signal, in cases where the renewal of the first tap coefficient is judged by the first adaptive filter renewal control means, and producing a first pseudo-echo signal from the first tap coefficient;
first subtracting means for subtracting the first pseudo-echo signal produced by the first adaptive filter means from the transmission signal to produce the first error signal as the echo canceled output signal equivalent to the transmission signal in which an echo component is canceled;
second adaptive filter renewal control means for receiving the reception signal and judging according to a second renewal judging method based on the reception signal whether or not a second tap coefficient is to be renewed;
second adaptive filter means for receiving the second error signal and the reception signal, renewing the second tap coefficient according to a second tap coefficient renewal algorithm, which is based on the second error signal and the reception signal and differs from the first tap coefficient renewal algorithm used by the first adaptive filter means, in cases where the renewal of the second tap coefficient is judged by the second adaptive filter renewal control means, and producing a second pseudo-echo signal from the second tap coefficient;
second subtracting means for subtracting the second pseudo-echo signal produced by the second adaptive filter means from the transmission signal to produce the second error signal equivalent to the transmission signal in which the echo component is canceled; and
adaptive filter selecting means for receiving the reception signal, the transmission signal, the first error signal produced by the first subtracting means, the second error signal produced by the second subtracting means and the second tap coefficient of the second adaptive filter means and controlling the first adaptive filter means according to a level relationship among the reception signal, the transmission signal, the first error signal and the second error signal to replace the first tap coefficient of the first adaptive filter means with the second tap coefficient.
In the above configuration, because the first tap coefficient is renewed according to the first tap coefficient renewal algorithm in the first adaptive filter means, the first tap coefficient is stably converged, and a deterioration degree of the first tap coefficient is reduced in a double talk state. In contrast, because the second tap coefficient is renewed according to the second tap coefficient renewal algorithm differing from the first tap coefficient renewal algorithm in the second adaptive filter means, though a deterioration degree of the second tap coefficient is high in the double talk state, the second tap coefficient is rapidly converged in a state in which only the echo component exists in the transmission signal.
Accordingly, in the echo canceling apparatus, the echo canceled output signal equivalent to the transmission signal, in which the echo component is canceled according to the highly estimated tap coefficient relating to the production of the echo canceled output signal, is always produced and output regardless of an echo path change or an infinite return loss. Also, the echo canceling apparatus not depending on an echo path environment can be manufactured in a small size without amplifying the echo component included in the transmission signal. Also, the echo canceling apparatus, in which the echo canceled output signal is output without any delaying time, can be obtained.
It is preferred that a first tap coefficient renewal algorithm used for the renewal of the first tap coefficient of the first adaptive filter means differs from a second tap coefficient renewal algorithm used for the renewal of the second tap coefficient of the second adaptive filter means.
Therefore, the renewal of the tap coefficient corresponding to the production of the echo canceled output signal is not stopped regardless of the echo path change or the infinite return loss. Also, the echo canceling apparatus not depending on an echo path environment can be manufactured in a small size without amplifying the echo included in the transmission signal. Also, the echo canceled output signal is output without any delaying time.
It is preferred that the adaptive filter selecting means receives the first tap coefficient of the first adaptive filter means in addition to the reception signal, the transmission signal, the first error signal, the second error signal and the second tap coefficient, and the adaptive filter selecting means controls the first adaptive filter means according to a first level relationship among the reception signal, the transmission signal, the first error signal and the second error signal to replace the first tap coefficient of the first adaptive filter means with the second tap coefficient or controls the second adaptive filter means according to a second level relationship among the reception signal, the transmission signal, the first error signal and the second error signal to replace the second tap coefficient of the second adaptive filter means with the first tap coefficient.
Therefore, the convergence speed of the second tap coefficient in a time-period after the double talk state can be moreover heightened.
It is also preferred that the adaptive filter selecting means controls the first adaptive filter means to replace the first tap coefficient of the first adaptive filter means with the second tap coefficient according to a linear interpolation based on a time transition.
Therefore, the discontinuity occurring in the echo canceled output signal in the replacement of the tap coefficient can be reduced.
It is also preferred that the echo canceling apparatus further comprising:
error signal buffer control means for receiving a judging result indicating the replacement of the first tap coefficient of the first adaptive filter means with the second tap coefficient or a judging result indicating the replacement of the second tap coefficient of the second adaptive filter means with the first tap coefficient from the adaptive filter selecting means and producing a first multiplication factor and a second multiplication factor according to the received judging result;
first multiplying means for multiplying the first error signal produced by the first subtracting means by the first multiplication factor produced by the error signal buffer control means to produce a first multiplied error signal;
second multiplying means for multiplying the second error signal produced by the second subtracting means by the second multiplication factor produced by the error signal buffer control means to produce a second multiplied error signal; and
adding means for adding the first multiplied error signal produced by the first multiplying means and the second multiplied error signal produced by the second multiplying means together to obtain a sum of the first multiplied error signal and the second multiplied error signal and outputting the sum as the echo canceled output signal.
Therefore, the discontinuity occurring in the echo canceled output signal in the replacement of the tap coefficient can be reduced while reducing a calculation volume required in the tap coefficient replacement.
It is also preferred that a value of a convergence factor used for the renewal of the first tap coefficient of the first adaptive filter means differs from that used for the renewal of the second tap coefficient of the second adaptive filter means.
Therefore, the echo canceled output signal, in which the echo component is reliably canceled, can be moreover stably obtained. Also, the convergence speed of the second tap coefficient can be moreover improved in the only echo component state, even though the echo path change or the infinite return loss occurs, the estimate of the second tap coefficient can be moreover rapidly performed, and the echo canceled output signal, in which the echo component is removed from the transmission signal, can be always obtained stably regardless of the echo path change or the infinite return loss.
It is also preferred that a length of the first tap coefficient of the first adaptive filter means differs from that of the second tap coefficient of the second adaptive filter means.
Therefore, the echo canceled output signal, in which the echo component is reliably canceled, can be moreover stably obtained. Also, the echo canceled output signal, in which the echo component is removed from the transmission signal, can be always obtained stably regardless of the echo path change or the infinite return loss.